Fix compiler warnings

- implicit declaration of function ‘isprint’ [-Wimplicit-function-declaration]
- comparison between ‘result_t’ and ‘enum <anonymous>’ [-Wenum-compare]

impl/src/layer1/rx.c

@@ -454,7 +454,7 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 									payload_enc, 8, sizeof(payload_enc), &nsyms));
 
 						fec payload_fec = modcod_create_fec(rx->modcod);
-						result_t res = fec_decode(payload_fec, rx->payload_len_bytes, payload_enc, payload);
+						int res = fec_decode(payload_fec, rx->payload_len_bytes, payload_enc, payload);
 						fec_destroy(payload_fec);
 
 						if(res != LIQUID_OK) {

impl/src/utils.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <ctype.h>
 
 #include <errno.h>
 #include <time.h>